We have developed optical system for monitoring the membrane potential from contracting preparations such as heart and muscle. In optical signal recorded from such preparations, the light-scattering component derived from the contraction is overlapped to the component of membrane potential change. It is possible to separate these two components using the difference in wavelength dependency. In dual wavelength recording from many regions simultaneously, the chopping method for switching the wavelength of the incident light has great advantages compared with that for dividing the optical path by dichroic mirror. But the switching speed, in conventional mechanical method, is not fast enough for heart or muscle preparation. Introducing ultra-high luminance light emitting diode into the light source made it possible to develop the recording system with measurement by high speed dual wavelength chopping. First two years, we made a copy of 1020-site optical recording system in Tokyo Medical and Dental university, which Hirota (Head investigator) developed. It is basically same, but the continuous recording time was expanded from one second to several tens of minutes. Second, we developed a light source using ultra-high luminance light emitting diode, and the remodeling the recording system for fitting the light source chopping. Using this new system, we succeeded in recording an excitation wave spreaded on the rat atrium in oval form. Since the cell under each element had been replaced with the big beating, the beating had to be suppressed, but using this dual wavelength simultaneous recording system, it is easy to automatically detect the time for the excitement beginning in each pixel using computer software.